The present invention generally relates to reference signal detection circuits in rotary recording medium reproducing apparatuses, and more particularly to a detection circuit capable of detecting reference signals for use by a tracking servo system, the reference being reproduced in a burst-like manner from a rotary recording medium, without introducing aliasing noise, by use of a simple circuit construction.
Systems have been realized in which a recording system forms pits in accordance with information signal being recorded along a spiral track on a flat rotary recording medium (hereinafter referred to as "disc"), without forming a groove therein, and a reproducing stylus traces over along this track to reproduce the recorded information signal in response to variations in the electrostatic capacitance in a reproducing system.
In this system, since no groove is provided on the disc for guiding the reproducing stylus, pilot or reference signals for enabling a tracking servo control are recorded on or in the vicinity of a track of the information signal, such as a video signal, on the disc. Upon reproduction, the reference signals are reproduced together with the video signal. Tracking servo control is carried out so that the reproducing stylus accurately traces along the track in response to the reproduced reference signals.
In the above disc, only one of a first and second reference signals fp1 and fp2 is recorded at an intermediate position between center lines of adjacent track turns. Moreover, the side of which the first and second reference signals are recorded with respect to one track turn changes over every one track turn. That is, when the first and second reference signals are respectively recorded on the right and left sides of one track turn, the relationship between the recorded positions of the reference signals is such that the second and first reference signals are respectively recorded on the right and left sides of adjacent track turns. Furthermore, a third reference signal for obtaining a changeover signal upon reproduction, is recorded for every track turn at recording changeover positions of the above first and second reference signals.
In a reproducing apparatus, a changeover operation is performed by use of the third reference signal reproduced upon obtaining of a tracking servo signal from the reproduced first and second reference signals.
Since no grooves are provided in the above disc, the reproducing stylus can be transferred from one track to another without damaging the reproducing stylus or the disc. Accordingly, in addition to the special reproduction such as still reproduction, slow-motion reproduction, and quick-motion reproduction, the system is capable of performing a so-called random access in which the reproducing stylus is transferred to a desired position at high speed to reproduce the desired information.
The above first and second reference signals fp1 and fp2 are recorded in a burst-like manner for every period corresponding to the horizontal blanking period of the video signal which is recorded as the information signal. Hence, in the apparatus for reproducing the above signals recorded in the disc, a detection circuit for detecting these first and second reference signals for tracking servo conventionally consisted of a circuit for rectifying and smoothing input reference signals, and a sample-and-hold circuit having a switching element for sampling the rectified and smoothed signal for every horizontal scanning frequency of 15.75 kHz and a capacitor for holding the sampled signal.
However, since the above conventional detection circuit consists of the circuit for sampling a signal at a frequency of 15.75 kHz, in a case where noise having a frequency of 7 kHz, 14 kHz, and the like is mixed into the signal which is to be sampled, the frequency difference between the above sampling frequency of 15.75 kHz and the noise frequency of 7 kHz, 14 kHz, and the like, is introduced as aliasing noise. When the above aliasing noise frequency is a frequency near the gain cross-over frequency of the tracking servo system, that is, near the frequency where the open-loop again becomes 0 dB, the tracking servo system becomes incapable of performing stable operation due to the aliasing noise frequency since the improvement factor in the vicinity of the gain cross-over frequency is exceedingly small. Therefore, there was a disadvantage in that an accurate tracking servo operation could not be performed.